Modern magnetic resonance systems generally operate with a plurality of different antennas (called coils in the following) for transmitting radiofrequency pulses in order to excite nuclear resonance and/or for receiving the induced magnetic resonance signals. Normally, a magnetic resonance system possesses a relatively large whole-body coil which is permanently installed in the device. The whole-body coil is typically arranged in the shape of a cylinder—e.g. having a so-called “birdcage” structure—around the patient receiving chamber in which the patient is positioned on the patient support table during the measurement. One or more small local coils or surface coils are frequently used in addition in a tomography apparatus.
For more extensive examinations, a plurality of coil arrays (multicoil receiver arrays), each including a number of interconnected coils, are often even placed on and/or under the patient. These local coils serve for acquiring detailed images of a patient's bodily parts or organs which are located relatively close to the body surface. For this purpose the local coils are applied directly at the point on the patient at which the region to be examined is located. When such a local coil is used, pulses are in many cases transmitted by way of the whole-body coil (as transmit coil), and the induced magnetic resonance signals are received via the local coil (as receive coil).
In order to generate good-quality magnetic resonance images it is undoubtedly important to select, from the plurality of coils present in the device, precisely the coils that are particularly suitable for a specific measurement of a specific measurement region, i.e. for example a specific slice or a stack of slices or, as the case may be, a volume within the measurement object. This has hitherto been done manually in the prior art by inputting corresponding selection commands at a control terminal of the tomography apparatus. In this case the operator makes a selection according to whether the coil in question is located in a suitable position relative to the region that is to be imaged in the following measurement and has an appropriate illumination zone, i.e. whether the region of interest can be measured at all by way of the coil.
For coils having a fixed position relative to the patient support table, this position is in some case specified explicitly ex works. This position is then essentially known to the magnetic resonance system, i.e. in the control device of the magnetic resonance tomography apparatus, even if the coil can usually be displaced within a small range. Alternatively, the position can for example also be measured explicitly prior to the magnetic resonance measurement. Often only the position in the z-direction, i.e. in the longitudinal direction of the patient support table, is usually measured in this case. The coordinates perpendicular thereto continue to be unknown as previously and are estimated at a likely average value ex works by certain manufacturers. An illumination zone can also be specified ex works for each coil. In this case, however, this is simply an estimated illumination zone that is to be expected on average. In particular it is not taken into account in this case whether the zone is also actually filled by a load during a measurement or whether the illumination zone has a shape due to the load that is quite different from the specified shape, for example a rectangle.
The correct choice of coils accordingly requires a considerable degree of knowledge and experience on the part of the operator, in particular because in practice the available information specified ex works in relation to positions and illumination zones of the coils is often not sufficiently precise and does not take into account the real circumstances for the live measurement. If the optimal coil or coil combination is not chosen for a subsequent measurement, then the quality of the subsequent image acquisitions will inevitably also deteriorate. In certain cases this may lead to the need to repeat the image acquisitions once more, which increases the total acquisition time. This not only reduces the efficiency of the magnetic resonance tomography apparatus and of the operating staff, but above all results also in the patient being exposed to a higher load.
A further reason for acquiring images of individual organs of patients via magnetic resonance tomography using a receive coil system comprising a plurality of coils, a so-called “multicoil receiver array”, is the faster image acquisition. Methods such as parallel imaging, for example, find application in this case. This explains the trend toward equipping MRT systems with more and more RF coils. It is therefore particularly important to safeguard the quality of the acquired images in order not to risk a repetition of the acquisitions, since in such a case the time advantage of the multicoil receiver arrays would be forfeited again. While it is true that the majority of the coil elements contribute toward improving the signal-to-noise ratio (SNR), the coils arranged further away from the target region of the image acquisition cause an increase in the noise level, since for these the greatest signal contribution comes from regions of the patient that do not belong to the target region. With radial imaging especially, a large number of peripheral coils can lead to increased streaking artifacts, in particular in the case of image planes in the longitudinal axis. It may therefore be beneficial to deactivate some of the coils prior to the measurement. It is, however, difficult to establish individually in advance for a specific object, which coil elements will produce artifacts and/or intensify the noise.
A possible approach is to determine the most suitable coils for a particular target organ in a post-processing step by visual inspection. Although this method can be very precise, it is time-consuming and cannot be applied to a real-time reconstruction because the selection of the best coils must be made retrospectively. If a predefined, fixed set of coil elements selected prior to the image acquisition and not dependent on actual individual conditions is used, it is probable that the set is not optimized to the particular examination subject.
In a conventional method for reducing noise and the signal-to-noise ratio, a linear combination of the coil elements is used in order to optimize the signal-to-noise ratio of the image of the target organ that is to be acquired, based on the sensitivity characteristics and noise statistics of the receiver coils. In another conventional approach, the case of radial imaging in particular is considered, where the extent of the streak artifacts in the images of each individual coil is evaluated by comparing them with a filtered, low-resolution image.
In yet another alternative approach, the signals of the different coil elements are combined by taking into account the specific hardware characteristics and the position of the object that is to be imaged. For example, in the acquisition of images of the heart, it is taken into account that the heart is positioned close to the center of the bodycoil. However, the two latter approaches in particular cannot be applied in a generalized manner. The former method requires a high computational overhead and occupies a proportionately long amount of time, which reduces the level of comfort for patients. If it is simplified and for example does not take precisely into account the individual dimensions of the organ to be examined, the result is not optimal.